Tuning the Photocatalytic Activity of Ti-Based Metal–Organic Frameworks through Modulator Defect-Engineered Functionalization

Defect engineering is a valuable tool to tune the photocatalytic activity of metal–organic frameworks (MOFs). Inducing defects through the attachment of functionalized modulators can introduce cooperative units that can tune the bandgap of the material and enhance their chemical, thermal, and photostabilities among other properties. However, the majority of defect engineering studies for photocatalytic applications are limited to Zr-based MOFs, and there is still a lack of interrelation between synthetic variables, the resultant MOF properties, and their effect on their photocatalytic performance. We report a comprehensive study on the defect engineering of the titanium heterometallic MOF MUV-10 by fluoro- and hydroxy-isophthalic acid (Iso) modulators, rationalizing the effect of the materials’ properties on their photocatalytic activity for hydrogen production. The Iso-OH modified MOFs present a volcano-type profile with a 2.3-fold increase in comparison to the pristine materials, whereas the Iso-F modified samples have a gradual increase with up to a 4.2-fold enhancement. It has been demonstrated that ∼9% of Iso-OH modulator incorporation produces ∼40% defects, inducing band gap reduction and longer excited states lifetime. Similar defect percentages have been generated upon near 40% Iso-F modulator incorporation; however, negligible band gap changes and shorter excited states lifetimes were determined. The higher photocatalytic activity in Iso-F modulator derived MOF has been attributed to the effect of the divergent defect-compensation modes on the materials’ photostability and to the increase in the external surface area upon introduction of Iso-F modulator.


S.1. General Experimental Remarks
Powder X-Ray Diffraction (PXRD): PXRD patterns were collected in a PANalytical X'Pert PRO diffractometer using copper radiation (Cu Kα = 1.5418 Å) with an X'Celerator detector, operating at 40 mA and 45 kV. Profiles were collected in the 3° < 2θ < 40° range with a step size of 0.017°. (University of Valencia) Thermogravimetric Analysis (TGA): were carried out with a Mettler Toledo TGA/SDTA 851 apparatus between 25 and 800 °C under ambient conditions (10 °C·min −1 scan rate and an air flow of 9 mL·min−1). (University of Valencia) Nuclear Magnetic Resonance Spectroscopy (NMR): NMR spectra were recorded on either a Bruker AVIII 300 MHz spectrometer and referenced to residual solvent peaks. (University of Valencia) Gas Uptake: N2 adsorption isotherms were carried out at 77 K on a with a Micromeritics 3Flex gas sorption analyser. Samples were degassed under vacuum at 120 °C for 24 h in a Multisorb station prior to analysis. BET surface areas, micropore surface areas and external surface areas were calculated from the isotherms using the MicroActive operating software. The pore size distributions were calculated using DFT cylindrical pore oxide surface model within the MicroActive software,(University of Valencia) Scanning Electron Microscopy (SEM) and single point energy-dispersive X-Ray analysis (EDX): particle morphologies, dimensions and mapping were studied with a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 20 kV, over metalized samples with a mixture of gold and palladium during 90 seconds. (University of Valencia) Fourier Transform Infrared Spectroscopy: IR spectra of solids were collected using a Shimadzu Fourier Transform Infrared Spectrometer, FTIR-8400S, fitted with a Diamond ATR unit. (University of Valencia) UV-Vis diffuse reflectance spectroscopy (DRS) was performed on a Jasco V-670 spectrophotometer using an integrated Labsphere in the range 200-800 nm. (University of Valencia) Photocatalytic experiments: H2 evolution upon UV-Vis irradiation with a 300 W Xe lamp was tested by dispersing 20 mg of the different samples in Milliq-H2O:MeOH mixtures (4:1, v:v) at 1 mg/mL in a quartz photoreactor, purging exhaustively with Ar before irradiation. Analysis of the evolved gases has been carried out through a micro-GC with two columns (Molsieve 5A and PorePlot Q) and TC detector that allowed us to monitor and quantify H2, O2, N2, CO2, among other gases. (Universidad politecnica de Valencia)

S.2. Materials and Synthesis
All reagents unless otherwise stated were obtained from commercial sources and were used without further purification.

General remarks
For all modulated syntheses a mixture of solvents (2.2 mL of AcOH per 9.6 mL of DMF) was prepared in function of the number of reactions to perform (11.8 mL per reaction). This pre-made solvent mixture was used to separately dissolve the different synthetic components as further explained during this section.
In all syntheses the jars were placed in an oven at room temperature and heated to 120℃ with 2℃/min ramp. The temperature was maintained during 24 hours and cooled down to room temperature with 0.4℃/min ramp. The resultant powder was collected by centrifugation (5000 rpm, 5 min) and washed with DMF (X2) and MeOH (x3) through dispersion-centrifugation cycles. The samples were dried under vacuum overnight and further activated by sohxlet with MeOH during approximately 24 hours. The samples were further dried under vacuum for 24 hours prior to characterization.
Procedure MUV-10-Iso-OH%: In 25 mL pyrex jars, CaCl2 (1 equivalent) was dissolved in 2 mL of solvent mixture. In a separate vial 1.5 equivalents of btc compared to Ti and Ca were dissolved in 9.8 mL of solvent mixture together with the modulator (number of equivalents compared to btc as desired depending on the synthetic conditions). Both solutions were mixed in in 25 mL pyrex jars followed by slow Ti(IV) isopropoxide addition (1 equivalent) and gentle stirring.

S.3.4. Proton Nuclear Magnetic Resonance ( 1 HNMR)
Iso-OH was present in the 1 HNMR profiles alongside formic acid coming from the decomposition of DMF during synthesis. Incorporation of the modulator and formic acid is expressed as the molar ratio (Rmod,) between modulator and btc, Rmod = Mod btc and as the molar percent of modulator (mol%) compared to btc, mol%= Mod Mod+btc * 100, while the total modulator percent (total mod%) is calculated taking into account modulator, formic acid and btc, total mod% = Mod+FA Mod+FA+btc * 100

S.3.6. Thermogravimetric analysis (TGA)
We have analysed the composition of MUV-10-Iso-X through combination of TGA with molar ratios determined by 1 HNMR, assuming that Iso-X is incorporated into MUV-10 structure TiCaO(H2O)W(BTC)X(ISO)y(FA)z(OH)D. 1 As Iso-X decomposes during the decomposition range of BTC, the experimental ratio between the molecular weigh of the DH MOF and its residue is expressed as follows:       Figure S31: Molar percent of missing linkers a function of the modulator added to synthesis. Figure S32: TGA profiles of MUV-10 synthesised with increased concentrations of Iso-F, compared to pristine MUV-10, with the start of the decomposition profiles normalised to 100%. A slight increase in metal content can be appreciated. Figure S33: TGA profiles of MUV-10 synthesised with increased concentrations of Iso-F, compared to pristine MUV-10, with the residues normalised to 100%. A decrease in the btc decomposition step is observed, with no decrease in thermal stability. Figure S34: Amplification of TGA profiles of MUV-10 synthesised with increased concentrations of Iso-F, compared to pristine MUV-10, with residues normalised to 100%. No decrease in thermal stability is observed.          Figure S44: Representation of the BET surface area as a function of the modulator added to synthesis.

S26
S33 Figure S45: Representation of the BET surface area as a function of the modulator incorporated (molar ratio compared to btc). Figure S46: Representation of the BET surface area as a function of the modulator incorporated (molar percent compared to btc) Figure S47: Representation of the BET and micropore surface area as a function of the modulator incorporated (molar ratio compared to btc) Figure S48: Representation of the micropore and total pore volumes as a function of the modulator incorporated (molar ratio compared to btc) Figure S49: N2 adsorption and desorption isotherms of MUV-10 synthesised with increased Iso-F concentration, showing an increase in porosity consistent with modulator addition and subsequent incorporation.

Methodology:
A quartz photoreactor attached with a regular manometer was loaded with 20 mL of a H2O:methanol mixture (4:1 v/v) in which 20 mg of MOF (1mg/mL) was prior dispersed in methanol. The reactor was purged with an Ar flow for 10-15 minutes, after which it was pressurized to 1 bar. The photocatalytic reactions were carried out by employing a focalized UV-Vis light generated by a 300 W Hg-Xe lamp (0.15 W/cm 2 ). All reactions were stopped after 24 hours of irradiation. No external heat source was employed. The evolution of H2 inside the photoreactor was monitored by the employ of an Agilent 490 MicroGC (MolSieve 5Å column, Ar carrier gas) with a TC detector. The system was calibrated prior to the experiments by injecting gas mixtures of known v%. Data quantification was carried out by employ of this calibration.

S.4.2. Photocalytic decarboxylation
The CO2 evolution of the samples was calculated using the maximum amount of possible CO2 released from the samples decarboxylation based on the composition obtained by thermal analysis (See Section S.3.6).

= 3 * + * − +
Then, the mass of CO2 in the 20 mg of samples employed during the photocatalytic studies is calculated, and the mass of CO2relases is normalised to that value. Table S29: Tabulated data for the CO2 evolution of MUV-10 pristine samples in µmol and normalised to the number of carboxylates present in the samples Table S30: Tabulated data for the CO2 evolution of MUV-10-Iso-OH in µmol.

S.5. Transient absorption spectroscopy
Transient absorption spectra were recorded using the forth harmonic of a Q switched Nd:YAG laser (Quantel Brilliant, 266 nm, 15 mJ/pulse, 7 ns fwhm) coupled to a mLFP-122 Luzchem miniaturized detection equipment. This transient absorption spectrometer includes a 300 W ceramic xenon lamp, 125 mm monochromator, Tektronix TDS-2001C digitizer, compact photomultiplier and power supply, cell holder and fibre-optic connectors, computer interfaces, and a software package developed in the LabVIEW environment from National Instruments. The laser flash generates a 5 V trigger pulses with programmable frequency and delay. The rise time of the detector/digitizer is ̴ 3 ns up to 300 MHz (2.5 GHz sampling). The monitoring beam is provided by a ceramic xenon lamp and delivered through a fibre-optic cable. The laser pulse is probed by a fibre that synchronizes the photomultiplier detection system with the digitizer operating in the pretrigger mode.